The prevalence of asthma among children of Puerto Rican ethnicity residing in New York City (NYC) has already been reported as among the highest in the world. In addition, we understand that housing factors influence levels of indoor allergens, such that poor housing lends rise to cockroach and mouse allergens, and high humidity is associated with high house dust mite (HDM) allergen levels. What is NOT known is the critical period of exposure in early life, the level of allergen exposure, and the duration of the exposure that leads to sensitization to indoor allergens, and how socioeconomic status, level of acculturation, and travel between NYC and Puerto Rico among these families influences this critical exposure. Our hypothesis is that Puerto Rican children living in NYC are exposed to more indoor allergens early in life than other children in NYC because they do travel to tropical environments where different types of dust mites are more abundant than in NYC. We will assess socioeconomic status, level of acculturation, travel between NYC and Puerto Rico, and the indoor allergen levels in their home environment in NYC and in the homes in Puerto Rico that are visited by them during the first 4 years of life among a birth cohort of Puerto Rican ethnicity from families where the mother has inhalant allergy living in NYC. At two timepoints, 2 and 4 years, we will collect blood from the child and measure IgE specific for dust mite, cat, cockroach, and mouse allergens. At the 4 yr clinic visit, we will also assess whether the child has a diagnosis of probable persistent wheeze asthma or other allergic diseases. Our assembled team includes an aeroallergen scientist, asthma and social epidemiologists, a pediatric pulmonologist, and a statistician, all of which are experienced in conducting large-scale, population-based studies. If we show that travel to Puerto Rico is associated with sensitization to HDM, will this deter parents from taking their children with them to the island? We hope not, because familial and cultural relations are important. This is where the blend of social and environmental science is crucial. We must understand how the two lead to allergic sensitization and be cognizant that they both will be required for the most effective primary prevention of allergic asthma.